1. Field
This application relates to methods and systems for calibrating a focus point for a camera lens.
2. Description of Related Art
High-end cameras and lenses are often susceptible to back-focusing or front focusing problems, which may be described as automatically focusing on a point that is slightly behind or in front of the point that the photographer intends to focus on, usually the surface of some object. These problems arise from manufacturing tolerances used in mass-production of camera lenses, coupled with the fact that, particularly for long lenses, any imprecision in the manufacture of the lens or its auto focus system may show up as a noticeable lack of focus, especially when automatically focusing on a distant object. Imprecision can also build up in unpredictable ways from coupling a lens to other elements in the optical path, such as a camera body.
To compensate for unavoidable imprecision in manufacturing, many camera manufacturers build cameras equipped with Micro-AF adjustment capability. Such capability enables the user to instruct the camera to automatically apply a degree of plus or minus focus correction in response to being coupled to a defined lens. It remains up to the user to determine an amount of focus correction, sometimes called a calibration amount, that should be applied for each particular camera and lens combination.
Various methods may be used to determine an amount of focus correction for a particular camera/lens combination. For example, “LensAlign”™, available from www.whibalhost.com, is a device that may be used to measure an amount of back focus or front focus, and thereby determine an amount of focus correction to apply. A LensAlign device is comprised of a base for holding the camera which can sit flat on a table, or be mounted on a light stand or tripod, together with a front standard and a rear standard. The front standard provides a focus target and is marked with precision rulings similar to a measurement ruler or the like. The rear standard is marked with several bulls-eye targets that line up with holes on the front standard, for aligning the camera with the front standard so that the camera and focus target are exactly plane parallel. Alignment is performed by sighting through the camera and lens until the bulls-eye targets are exactly aligned with holes in the front standard, then using the camera's autofocus to focus on the front standard, capturing an image and evaluating the image. Because of the precision rule markings on the front standard, the amount of front or back focus is readily visible from the captured image. Further details are shown at http://www.whibalhost.com/lensalign/how-lensalign-works.html.